The invention relates generally to a thermal inkjet printer; and more generally, to an optimum initial operating temperature for a thermal inkjet printer.
In designing a thermal inkjet printer, it is important to provide as economically and simply as possible a relatively high output quality at a relatively high speed. The output quality and relative speed of a thermal inkjet printer are often times a function of the startup operating temperature of the printhead, especially after a period of non-use.
For example, conventional thermal inkjet printers contain multiple inkjet nozzles. Associated with each nozzle is a heating resistor and a drive transistor. The nozzle includes a nozzle chamber within which the heating resistor is located. To fire ink from the nozzle chamber, the drive transistor outputs a firing pulse to the heating resistor. The firing pulse is a current pulse of a magnitude sufficient enough to heat up the resistor and thus the ink to an ejection temperature. The ink then ejects from the chamber toward a print media sheet. To determine when any given nozzle is to fire, a controller circuit is used.
Typically, existing printers use a single print head operating temperature throughout the duration of printing a document. If this temperature is set too high, then a variety of longer term reliability issues can occur such as ink plugs in the nozzles, material degradation in the print head, or ejection of overly concentrated colorant from evaporation of the ink vehicle thought the nozzles. If this temperature is set too low, then there can be significant initial short term reliability issues with getting the print head to reliably fire when first called upon to do so. What is needed is high initial ejection reliability of high initial operating temperatures combined with the improved long term reliability afforded by lower operating temperatures for the duration of image.
In certain printers, to maximize reliable ink drop ejections, the ink is pre-heated. However, to pre-heat the ink when the printer is not is use would result in a waste of energy and ink as the ink will thicken or be reduced through evaporation. Furthermore, because of ink evaporation, pre-heating the ink during a long period of non-use may damage the printhead. For all these obvious reasons, therefore, the resistors are not pre-heated if the printer is not in use.
It is well known in the industry that one of the problems associated with thermal inkjet printers concerns the amount of ink ejected or deposited from the printhead during the formation of each ink drop. The quantity of deposited ink, commonly referred to as the xe2x80x9cdrop-volumexe2x80x9d of the printhead, is dependent on the temperature of the printhead. If the printhead is cool, it will deposit less ink in each droplet. Missing, weak or low drop-volume results in poor quality images that appear faint or washed out. Consequently, when a printer has gone through a period of non-use or the printhead is cool, a certain amount of firing time is required to allow the printhead to reach its optimum drop-volume. This is usually accomplished by having the nozzles spit or eject low drop-volume ink droplets into a spittoon. Obviously, this scheme fosters ink wastage and a longer printing time.
Therefore, what is needed is a method to facilitate a thermal inkjet printer to reach its optimum drop-volume from a period of non-use as quickly as possible while minimizing ink wastage.
To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention is embodied in a printing system for improving the edge sharpness, color uniformity, banding and faint or washed out appearance of ink drops produced by an inkjet printer.
The need in the art is addressed by the present invention. The present invention provides a thermal inkjet printer with the requisite technology to increase or reduce its operating temperature. The printer uses a sensor to detect the operating temperature of its printhead. If the temperature of the printhead is below the printhead""s default or normal operating temperature when the printer is going to start to print an image or document, the operating temperature of the printhead is set at a temperature higher than its default or normal temperature.
This is to ensure that the drop-volume of the printer stays at an optimum level when the printer is starting to print the image or document after a period of non-use. Shortly after the printer has started the printing task, the operating temperature of the printhead is reduced to its default normal operating temperature. The higher temperature depends on the probability of successful ejection of the nth drop. Satisfactory image quality depends on all drops to have the proper volume, velocity and directionality.
The present invention as well as a more complete understanding thereof will be made apparent from a study of the following detailed description of the invention in connection with the accompanying drawings and appended claims.